In summary, HCC is the third most common cause of cancer mortality and has the fastest growing mortality rate of any cancer in the US. There are limited treatment options for this cancer and despite the advance of modern chemotherapy, the long-term outlook for patients with liver tumors remains extremely poor. Intensive research efforts have been directed toward the identification of the mechanism of initiation and progression of HCC in hopes of developing biomarkers for early diagnosis and novel treatment strategies. In order to develop rational immunological strategies for treating liver malignancies, it is important to characterize the underlying host-tumor interactions. Our preliminary data suggests that there is an ineffective immune response to HCC, which is characterized by impaired CD response, increased populations of circulating CD4+CD25+ FoxP3 regulatory T cells (Treg) and soluble CD25. We found an elevation of Tregs with enhanced suppressive function in the peripheral blood of patients with HCC which correlated with stage and tumor burden of HCC. In addition, there was an overall level of T cell suppression (based on CD4 T cell proliferation assays) and a close relationship with soluble CD25 and overall tumor burden. Soluble CD25 represents a potential simple, ELISA-based biomarker in the serum of T cell activation (sCD25 is cleaved from T and NK cells on activation) and because recent evidence has suggested a role in tumor related immunosuppression. In this application, we will address the hypothesis that Tregs and CD25 are increased in peripheral blood of HCC patients and promote HCC development/progression via suppression of cell mediated CD4 and CD8 T cell responses, and depletion of Tregs and/or sCD25 would enhance cellular immunity against HCC. In Specific Aim 1 we will directly evaluate the mechanisms of sCD25 induction by human HCC and modulation of immune response. In Specific Aim 1, we will also evaluate our hypothesis in patients who develop HCC and will correlate the sCD25 levels and Treg frequency / function with HCC development, progression, and response to therapy. Our proposed study will provide critical insights into the function of Tregs in liver cancer. More importantly, using a well-characterized HCC patient cohort and a well characterized animal model, we will be able to develop novel diagnostic and therapeutic strategies for HCC. In Specific Aim 2, we will characterize the expression of TRAIL and its death receptor DR4 and DR5 in human HCC tissues, determine the efficacy of TRAIL agonists on human HCC cells cultured from resected cancer tissues, and initiate clinical trials in patients with HCC using HGS-ETR. This study (initiated based on our own research data) will be the first in the world to comprehensively investigate the therapeutic potential of TRAIL agonists in human liver cancer. The outcome of our in-vitro and in-vivo studies will provide critical insights into the global effect of TRAIL agonists on human organ systems, which is essential data for broad clinical applications.